We demonstrate the fusion of giant unilamellar POPC
liposomes brought together by the use of optical tweezers. The liposomes are
formed by the electroformation method. Prior to the electroformation process
DIOC dye is mixed in with the lipids. During electroformation DIOC dye attaches
to the membranes of the forming liposomes. A solution of sucrose is added to
the electroformation chamber and the liposomes enclose this solution as they
form. The liposomes are about 10-50 micrometers in diameter. Since it is very
hard to optically trap giant unilamellar liposomes the sucrose solution helps
increase the trapping ability by changing the index of refraction of the
vesicles. After the liposomes are formed they are collected in a flask and
mixed in with HEPES buffer. The sucrose solution also makes the vesicles
heavier, giant liposomes are collected from the bottom after a few minutes of
low speed centrifuge of the flask.

After two liposomes are brought together by dual optical tweezers, the
fusion process is initiated by a UV pulse. We excite the DIOC dye on the
membranes of the liposomes and image the process fluorescence microscopy. The polarization
of the excitation light is continually varied during the process in order to
observe the orientation of the lipids on the membranes, before, during and
after fusion. Our main aim is to demonstrate whether a stalk forms during the
fusion process or not. Details of our optical setup, computer control and
various challenges that we have overcome will be reported.